This invention relates to a hermetically sealed refrigeration compressor, and in particular, to one having a pressurized cylinder block.
In a conventional type of sealed compressor unit used in refrigerating systems, the unit is mounted with the crankshaft extending vertically. The lower portion of the cylinder block is immersed in a supply of oil, or other lubricant, contained in a sump defined by the bottom wall of the outer sealed casing or shell. Adequate lubrication is supplied to the moving parts of the compressor by an oil pump, the oil returning to the oil sump through a drain opening in the bottom wall of the cylinder block. The cylinder block is closed, except for the drain opening and is therefore pressurized because of piston "blow-by gas," that is gas at discharge pressure which has by-passed the sealing means between the compressor pistons and cylinders to pressurize the cylinder block. An example of a compressor of this type is illustrated in U.S. Pat. No. 3,008,628. Generally, the lubricating oil freely drains through the opening to the sump since the sump is at suction pressure. That is to say, the pressure within the hermetically sealed shell is suction pressure and it is this pressure which acts on the surface of the oil stored in the sump.
Compressors of the type described may be employed in heat pump applications. Typically, in the heating mode of operation, it is necessary to defrost the outdoor coil then functioning as a refrigerant evaporator. Hot gas at discharge pressure is typically employed to defrost the outdoor coil. The hot gas returns to the suction side of the compressor thereby substantially increasing the pressure acting on the surface of the oil stored in the sump. In fact, the pressure acting on the surface of the oil may exceed the pressure in the cylinder block resulting from the blow-by gas. This can result in a reverse flow of oil through the drain opening in the bottom of the cylinder block. That is, the oil may now flow from the sump into the cylinder block, creating an excess quantity of oil in the block.
This excess quantity of oil in the cylinder block can increase power consumption and prevent proper seating of the piston rings, with the result that the oil will be pumped past the rings into the refrigerant discharge line, which per se is not desirable and whereby the volume of the oil stored in the sump may be reduced so that there is an insufficient amount of oil pumped to the moving parts of the compressor.